You can check back for awhile to see if anyone gives you different information or advice. I am a member of the VDC.
I have seen very little information here at the VDC on the subject. Speculation is that low Vitamin D levels may cause it. Since Guillain-Barre is an “autoimmune” condition, research is showing a clear link between Vitamin D deficiency and any condition/disease that is autoimmune.
I would be following my doctor’s advice on what to do. With that being said Vitamin D should only help and not hurt. Perhaps even shorten treatment time. If the patient is not taking any Vitamin D start there (5000iu) and make sure they are taking ALL cofactors. http://www.vitamindcouncil.org/about-vitamin-d/vitamin-d-and-other-vitamins-and-minerals/
You really did not say how long ago they were diagnosed or how long they have been treated for which may also play into any advice.
More questions, just ask!